1. Field of the Invention
The present invention relates to a digital data communication control technique in an apparatus such as an exposure apparatus.
2. Description of the Related Art
The dramatic development of semiconductor device technology in recent years has propelled significant improvements of the performance of various information apparatuses containing semiconductor devices. The development is supported by semiconductor manufacturing technology. Semiconductors are typically manufactured by using a semiconductor exposure apparatus called a stepper or scanner.
A stepper reduces an image of a pattern formed on a reticle and projects it onto a semiconductor wafer placed on a wafer stage under a projection lens to transfer pattern images onto multiple parts of the single wafer one after another through exposure while moving the wafer stepwise.
A scanner irradiates a semiconductor wafer placed on a wafer stage and a reticle placed on a reticle stage with a slit-shaped exposure light ray to project a pattern image formed on the reticle onto the wafer while moving (scanning) the semiconductor wafer and the reticle with respect to a projection lens.
The scanner is in widespread use as an exposure tool for a semiconductor device today because it surpasses the stepper in resolution and overlay accuracy. In the scanner, the wafer stage and reticle stage must be controlled to bring them into synchronization with each other with high accuracy. For this purpose, a digital control system is provided in the scanner.
The digital control system requires “hard realtime” capability, which is the capability to perform control interrupt processing at predetermined sampling periods without fail. In order to scan the wafer stage and the reticle stage with high synchronization accuracy, the scanner requires a multi-axis digital control system capable of providing multiaxial (e.g., more than 20 axes) control.
Furthermore, the control sampling periods significantly affects control performance. Therefore, it is desirable that the periods be as short as possible. The scanner requires control sampling periods of the order of 100 microseconds. Such a multiaxial fast control system is typically implemented by a digital control system with a multiprocessing configuration using multiple DSPs (Digital Signal Processors).
In addition to the hard realtime capability, the digital control system must have the capability of receiving a command from a user, providing the command to each DSP, and presenting the result of the execution of the command to the user. Furthermore, the digital control system must include “host capabilities” such as the capability of presenting a graphical representation of the status of a controlled object. User commands includes commands to activate and deactivate servo operation of a controlled object, to move a controlled object, and to change control parameters.
Processors that are specially designed for signal processing such as DSPs are not suitable for implementing these host capabilities. RISC CPUs, which are suitable for implementing host capabilities, are often used. Desirably, hardware that implements the host capabilities may be hardware on which a versatile OS having host-program library resources such as graphical user interface and TCP/IP functions can be run without needing porting. Therefore, general-purpose processor boards using basically a standard multi-drop bus such as a VME bus are widely used as the hardware that implements host capabilities. The general-purpose processor board enables efficient development of software for host capabilities exploiting various libraries of a versatile OS on the basis of hardware-dependent firmware provided by board manufacturers.
In order to implement a fast digital control system, fast control data communications between processors and between the processors and I/O devices (such as sensors and D/A converters) must be provided in addition to the fast throughput of control processors.
In order to transfer control data required for such control processing, typically it is desirable to use DMA transfer. This is because the DMA transfer is advantageous in that: (1) it does not place a significant load on processors during data transfer, and (2) it is faster than non-DMA transfer.
However, among the multi-drop buses, VME buses, which are widely used in the field of industrial machinery, provides a low bus-transfer rate and does not have the DMA transfer function, therefore is not suitable for control data communication in servo systems. Furthermore, multi-drop buses, including VME busses, are inherently incapable of providing multiple communications concurrently. When control computation processing that requires high hard-realtime performance is distributed among multiple DSPs, data communication waiting time occurs due to a low transfer rate or bus occupation. As a result, data communication becomes a bottleneck of the entire system.
Therefore, specialized communication ports of DSPs are used to provide fast control data communications between the processors and between the processors and I/O devices, thereby eliminating the bottleneck relating to data communication.
FIG. 1 shows an exemplary configuration of a conventional digital control system in an exposure apparatus using multiple DSPs. In FIG. 1, the solid arrows represent control data communication performed at every control sampling period and the dashed arrows represent command data communications by a host capability performed in response to commands from a user.
In command data communication, command data from a host CPU of a host apparatus is transferred to DSP 1 through a bus such as a VME bus and provided from DSP 1 to DSP 2 and DSP 3. Then, each DSP perform processing according to the command data.
In control data communication, positional information about a controlled object is detected by a sensor in every control sampling period and transferred to DSP 2. The sensor data is communicated between DSPs to perform control computations such as target position, position deviation, and PID computations. The calculated values are transferred to a D/A converter through DSP 2. In the communication between the DSPs and between the DSPs and I/O devices, data are transferred through communication ports of the DSPs. The control computation processing is usually performed by using a hardware interrupt function of the DSPs in every control sampling period.
FIG. 2 shows timing of data communication and computation processing performed by processors according to the conventional art described with reference to FIG. 1. As time progresses as indicated by the downward arrow in FIG. 2, the processors synchronously perform processing.
With reference to FIG. 2, command data processing will be first described. The host CPU generates command data in response to a command from a user and sends the command data to DSP 1. DSP 1 transfers the command data to DSP 2 or DSP 3 (in this example, DSP 2), depending on the content of the command data. DSP 2 changes the status of a servo system to be dealt with in DSP 2 according to the received command data and returns reply data to DSP 1 waiting for the reply data from DSP 2. Similarly, DSP 1 returns reply data to the host CPU waiting for the reply data. With this process, processing for the single command is completed.
The command processing in DSP 2 and DSP 3 is performed during time periods other than interrupt processing time periods for control computation. Depending on the amount of computation for command processing, the command processing is separated into multiple parts by sandwiching interrupt processings as indicated by symbols “a” and “b” in FIG. 2. Japanese Patent Laid-Open No. H7-110975 discloses an example of the technique for performing servo operation by interrupt processing and performing other processing in other time periods.
Interrupt processing, namely control computation processing, shown in FIG. 2 will be described next. In DSP 2 and DSP 3 that perform control computation, the following control interrupt routines are activated at predetermined intervals.    (1) Sensor Data capturing    (2) Filtering to sensors    (3) Calculation of a target position    (4) Calculation of PID    (5) Transfer of the result of computation to D/A converter
The series of control computations described above are performed by multiple DSPs in parallel, thereby a digital servo system is configured. This processing must be performed at during every predetermined sampling period.
However, the conventional digital control systems have problems as described below.
In order to increase the throughput of a semiconductor exposure apparatus, a system has been devised in recent years that uses a twin-stage configuration including an exposure station and an alignment station to concurrently perform exposure and alignment measurement. In the system, data obtained by measurement in the alignment station must be transferred to the stage control system in the exposure station in real time to control the leveling of the stage in the exposure station. Accordingly, the amount of data communicated significantly increases compared with other traditional control systems.
With the miniaturization of devices, the need for control of synchronization of optical elements (mirrors and lenses) constituting an imaging optical system with the wafer stage and reticle stage, in addition to the control of those stages, has arisen. The reason is as follows. The attitudes of the optical elements must be controlled in real time in order to suppress aberration of the optical elements with a high precision in the miniaturized devices and the control of the attitude of the optical elements at the same time causes variations of the image focus position on the wafer. These variations must also be corrected in real time.
Thus, the exposure apparatus requires an optical element control system in addition to the stage control system. Furthermore, realtime communication between these control systems is required in order to synchronize the stage control system and the optical element control system.
To achieve such communication, there is a strong demand for a more flexible communication technique for providing fast control data communication between multiple units in a control system of an exposure apparatus. However, it is difficult to implement such a fast and complex communication capability by using fixed, one-to-one communication which is inherent in DSP used in the conventional digital control system described above.
Instead of the fixed, one-to-one communication, multiple control data communications may be performed through a multidrop bus. However, multiple communications cannot be performed at a time through the bus and therefore communication wait time occurs due to bus occupation. Furthermore, it is difficult to contain a stage control system including an optical element control system and an alignment measurement system in a single multidrop-bus-based chassis because of a limited number of slots.
In recent years, interconnect communication technologies such as RapidIO and PCI-EXPRESS for inter-processor communication and I/O communication have rapidly developed. At the core of the development are the advances in fast communication technologies of the order of gigabits.
FIGS. 3A and 3B show exemplary communication topologies using a switched-fabric switching circuit. In FIGS. 3A and 3B, processors 1-4 and I/O devices 1-4 are connected onto a switched fabric. The interconnections between the I/O devices and processors connected to the switched fabric can be dynamically switched. For example, the connection topology in FIG. 3A is dynamically changed to the topology shown in FIG. 3B. In both topology, up to four connections can be established concurrently and data transmission can be performed concurrently. Therefore, occurrence of transmission wait time is suppressed as opposed to conventional multidrop bus connections. By changing connections dynamically and fast in this way, desired transfer operation can be performed. The processors (including a processor that implements host capabilities) and I/O devices connected to a switched fabric are collectively called “communication nodes” hereinafter.
The switched-fabric communication technique has been standardized by the IEEE, is being used in industrial and consumer apparatuses, and is becoming less expensive. The technique is also advantageous in that it is faster than the conventional multidrop bus connections, that the bottleneck caused by bus occupation can be eliminated because communication between multiple points can be concurrently performed, and that a flexible system configuration can be implemented because the connection points can be dynamically changed. Furthermore, by distributing the components of the system over multiple racks and interconnecting them through a switched fabric, a larger-scale system can be built compared with conventional multidrop bus systems in which the maximum number of slots that can be provided in a chassis is limited.
For the reasons described above, it is desirable that a switched fabric communication technique be used in a digital control system of an exposure apparatus. This will provide the following effects:    (1) Flexible and fast communication can be provided not only for stage control but also for measurements of wafers and control of optical elements.    (2) Command data communication and control data communication, which have been conventionally performed using different protocols, are integrated into switched fabric communication so that simpler and faster software and hardware can be implemented.
However, an exposure apparatus requires the “hard realtime capability” of performing multi-axis control computation processing at a sampling rate of the order of 100 microseconds in every sampling period. Accordingly, control data input/output communication and communication between processors also must be performed in every control computation processing period of the order of 100 microseconds in such a manner that the digital control computations can be completed in that period.
Therefore, in order to provide a digital control system that can dynamically change connections using the switched fabric communication, switching and control data communication must be completed in a time period sufficiently shorter than the control computing period. The switching and communication must be completed even when communication of a large amount of command data from a user is performed asynchronously.
The hard realtime capability for such communication is a tough condition which is not required by a so-called best-effort multimedia processing system such as digital audio apparatuses free of constraints concerning communication processing or computation processing that must be satisfied.
It is desirable to use DMA transfer in data communication. However, DMA transfer is a transfer method in which data is transferred without interruption until transfer of a desirable amount of data is completed once the transfer is started. Therefore, when a large amount of command data is transmitted in a digital control system implemented by using a switched fabric in which switching between command data communication and control data communication is required, the transfer of command data may not be able to be completed before the next sampling period starts.
The present invention has been made in view of these circumstances and a feature of the present invention is to provide a communication technique that enables fast data communication in a digital control system of an exposure apparatus while ensuring hard realtime capability of the exposure apparatus.